present invention relates to a transmission system for transmitting MPEG (Moving Picture image coding Experts Group) data through an ATM (Asynchronous Transfer Mode) line and, more particularly, to an AAL5 (ATM Adaptation Layer type 5) jitter reduction method and apparatus for reducing packing jitter of an AAL5 and jitter in a regeneration data rate and system time clock.
In a conventional transmission system for transmitting MPEG data through an ATM line, the phase-locked loop (PLL) technique is used to reduce AAL5 reception jitter.
FIG. 5 shows an ATM line signal processing apparatus disclosed in, e.g., Japanese Patent Laid-Open No. 8-205103. Referring to FIG. 5, a cell interval extraction section 501 extracts the ATM cell interval from received input data. A cell interval generation section 505 generates a reference ATM cell interval using the clock of the regeneration data rate regenerated by a voltage-controlled oscillator (VCO) 504. A comparator 502 compares the two ATM cell intervals to output the cell interval error. A low-pass filter (LPF) 503 smoothes the cell interval error from the comparator 502 and controls the output clock interval of the VCO 504 using the error smoothing signal, thereby making the output from the comparator 502 zero.
FIGS. 6A to 6C show a packet reception operation disclosed in Japanese Patent Laid-Open No. 8-97837. This prior art uses the xe2x80x9cIEEE 1394xe2x80x9d frame format. FIG. 6A shows the encoder output on the transmission side. FIG. 6B shows the AAL5 terminal output on the reception side. FIG. 6C shows corrected data.
In this scheme, the position of a transport packet can be properly grasped from a sync time representing the start of a frame and a data block number capable of specifying the start of the packet, as shown in FIG. 6C. On the basis of this information, a program clock reference (PCR) as information associated with the system time clock output by counting system clocks on the decoder side, which are transmitted from the encoder side, is corrected to reduce the system clock jitter on the decoder side.
In an xe2x80x9cMPEG over ATMxe2x80x9d (transmission system) which has been examined in the ATM Forum, an AAL5 is constituted by two MPEG transport stream packets. FIGS. 4A to 4D show the AAL5 frame format. As shown in FIG. 4B, a CPCS-PDU payload is constituted by two 188-byte MPEG transport packets 401. An 8-byte trailer is added to generate AAL5 data 402. As shown in FIG. 4C, the data 402 is divided into 48-byte data 403. An ATM cell header is added to each 48-byte data, thereby generating an ATM cell 404 to be transmitted, as shown in FIG. 4D.
In the standard data transmission system based on xe2x80x9cMPEG and ATMxe2x80x9d, the reception apparatus receives an AAL5 data unit and then constructs the two MPEG transport packets. The two MPEG data are time-compressed and output as burst data. For this reason, jitter is generated depending on whether the transport packet is arranged before or after the AAL5. This jitter is called packing jitter.
Conventionally, when fixed rate transmission is to be performed using the ATM line, the PLL is used to regenerate the input data rate on the reception side, as shown in FIG. 5, and additionally, a buffer is used to obtain fixed rate data. However, in MPEG regeneration, the frequency of the system clock of the decoder is 27 MHz which is asynchronous with the input data rate. For this reason, a PLL and a buffer are further required. That is, the apparatus has two PLLs and two buffers, resulting in a redundant arrangement.
If the input data rate need not be regenerated, the MPEG data can be decoded by directly regenerating the system clock at 27 MHz. However, when the system clock is regenerated while keeping the AAL5 packing jitter included, this jitter may be superposed on the system time clock.
When a scheme as shown in FIGS. 6A to 6C is to be applied to the ATM as a method of reducing the packing jitter, a sync time representing the start of a frame, i.e., an AAL5 and a data block number representing the start of a transport packet are necessary. However, these pieces of information cannot be transmitted by xe2x80x9cMPEG over ATMxe2x80x9d.
It is an object of the present invention to provide an AAL5 jitter reduction method and apparatus for, when MPEG data is transmitted through an ATM line, reducing the AAL5 packing jitter on the reception side.
It is another object of the present invention to provide an AAL5 jitter reduction method and apparatus for, when MPEG data is transmitted through an ATM line, reducing jitter on the reception side without using any PLL and buffer to regenerate an input data rate at a predetermined rate.
It is still another object of the present invention to provide an AAL5 jitter reduction method and apparatus for, when MPEG data is transmitted through an ATM line, regenerating a system time clock having excellent jitter characteristics on the reception side.
In order to achieve the above objects, according to the present invention, there is provided an AAL5 jitter reduction method for an image data transmission system for converting MPEG data into ATM cells and transmitting the ATM cells through an ATM line, comprising the steps of obtaining an addition average value of system time clocks at timings for detecting starts of adjacent transport packets after AAL5 termination, and correcting a transport packet arrival time on the basis of the obtained addition average value.